1. Field of the Invention
The present invention relates to a liquid crystal display which is improved in viewing angle characteristics.
2. Description of Prior Art
As a display for electronic office system-devices such as a desk-top personal computer and a word processor, CRTs (cathode ray tube) have been employed so far. Recently, a liquid crystal displays (hereinafter referred to as LCD) are widely employed instead of the CRTs because of their small thickness, light weight and low power consumption. The LCD, however, does not have good display characteristics (especially enlarged viewing angle) equal to those of CRT. The LCD generally has a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. Most of LCDs use a twisted nematic liquid crystal. The operational modes of LCD using the twisted nematic liquid crystal are roughly divided into a birefringence mode and an optical rotatory mode.
A super twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) utilizing the birefringence mode uses a super twisted nematic liquid crystal showing a twisted angle of more than 90 degrees and having steep electro-optical characteristics. Such a STN-LCD, therefore, has an advantage of giving the display of a large volume by driving in time-sharing mode. However, the STN-LCD has disadvantages such as slow response (such as several hundred milliseconds) and difficulty in giving satisfactory gradation on display, and therefore its display characteristics are relatively poor, as compared with display characteristics of the following liquid crystal display utilizing an optically rotary mode and further provided with the known active-type elements (e.g., TFT-LCD and MIM-LCD).
In the TFT-LCD and MIM-LCD, twisted nematic liquid crystal showing a twisted angle of 90 degrees and having positive birefringence is employed for displaying an image. This is called LCD of an optically rotary mode (i.e., TN-LCD). The TN-LCD display mode shows rapid response (such as several tens of milliseconds) and high display contrast, and easily gives black-white display of high contrast. Hence, the optical rotatory mode has a number of advantages compared with the birefringence mode or other modes. However, the TN-LCD has disadvantages such as color or contrast on display varies depending upon viewing angle to a liquid crystal display, and its display characteristics are not comparable to display characteristics of a CRT.
In order to improve the viewing angle characteristics (i.e. to enlarge the viewable angle), arrangement of a phase difference film (optical compensatory sheet) between a pair of polarizing plates and TN liquid crystal cell was proposed, and various optical compensatory sheets have been proposed up to now. Arrangement of the optical compensatory sheet enlarges the viewing angle to some extent however the enlargement is not sufficiently satisfactory.
Recently, a liquid crystal cell showing an inherently enlarged viewing angle has been proposed (e.g., Japanese Patent Provisional Publication No. 7(1995)-84254 and U.S. Pat. No. 5,410,422). The liquid crystal cell has a liquid crystal layer comprising liquid crystal capable of forming bend orientation therein and is a self-compensatory cell in which the molecular major axis of liquid crystal is symmetrical with respect to a center line between the substrates of the liquid crystal cell. The liquid crystal cell shows inherently an enlarged viewing angle.
Further, an abstract (29a-SZC-20, Collected abstracts of Applied Physical Society, 42nd, Spring, 1995) has proposed a reflective type liquid crystal display employing HAN mode (Hybrid-aligned-nematic mode) cell which utilizes the upper side of the liquid crystal layer of the bend orientation cell.
These self-compensatory cells bring about inherently an enlarged viewing angle, which is not equal to that given by a CRT. The application for LCD provided with an optical compensatory sheet suitable for the self-compensatory cell (U.S. application Ser. No. 749,894) has been already filed.
In the above-mentioned TN-LCD and the self-compensatory LCD, the display is conducted by selectively applying an electric field (voltage) in a direction perpendicular to the substrates of liquid crystal cell of LCD. To apply electric field or not gives variation of a direction of a molecular major axis of liquid crystal between the directions almost parallel and perpendicular to the substrates. Thus, display of black and white is mainly done by variation of direction of a molecular major axis in a plane perpendicular to the substrate. Therefore, retardation in the liquid crystal layer of LCD greatly varies depending upon variation of viewing angle, which results in variation of display color and contrast depending upon viewing angle.
A display mode utilizing a method applying electric field in a direction parallel to the substrates, which is not the above method applying electric field in a direction perpendicular to the substrates, has been proposed. The display mode is described in, for example, Asia Display '95 (The Institute of Telvision Engineers of Japan & The Society for information Display, pages 577-580 and 707-710), and Japanese Patent Provisional Publication No. 7(1995)-261152, and is referred to as "IPS mode" (In-plane Switching).
The IPS mode described in Asia Display '95 (pages 577-580) is explained according to the description referring to FIG. 1.
FIG. 1 (A) shows the arrangement of liquid crystal molecules and the condition of light in LCD utilizing IPS mode (IPS mode LCD) under no application of electric field (off state), and FIG. 1 (B) shows the arrangement of liquid crystal molecules and the condition of light in IPS mode LCD under application of electric field.
In FIG. 1 (A), nematic liquid crystal molecules 16 are aligned parallel to the substrates 13, 17 (i.e., homogeneous alignment), the substrate 13 having electrodes 14 thereon. An area of the substrate shown in FIG. 1 (A) usually corresponds to one pixel. The liquid crystal molecules respond to an electric field (voltage) with being kept in the plane parallel to the substrates. The optic axis directions (i.e., major axis directions) of the homogeneously aligned nematic liquid crystal molecules are almost parallel to polarization axis of a polarizing plate 12. The polarizing plates 12 and 19 (polariser and analyzer) are set such that the axes are in a crisscross. The arrangement of the polarizing plates gives a pure black state under no application of electric field to the LCD, because light 11 incident upon the LCD is not polarized within a layer of the liquid crystal molecules (liquid crystal layer).
In the case that an electric field is applied in the direction parallel to the substrates, the major axes of liquid crystal molecules are deviated from the axis of the polarizing plate 12, as shown in FIG. 1 (B). As a result, when the light 11 incident upon the LCD is passed through the liquid crystal layer, phase retardation occurs in the light due to the different propagating rates of the extraordinary and ordinary rays. Therefore, the light 11 is polarized (by the phase retardation) to pass through the polarizing plate 19. Thus, light transmission of the LCD is increased with increase of electric field to give a white state. The reference number 18 indicates polarized state of light passed through the liquid crystal layer. The 45 degree deviation of the optic axis of the liquid crystal molecule brings about maximum transmittance.